It is known that wood chips, possibly pretreated by heat and/or chemicals, are defibered by so-called refiners. These refiners, however, are also utilized for refining cellulose and mechanical pulps of various types when the paper-forming properties of these materials are to be developed by mechanical treatment. All such defibering or refining have in common the desired mechanical treatment of the fibrous material during its passage through the refiner. This treatment is effected upon the fibrous material, after it is fed into the refiner, by various means, whereby it leaves the refiner through a narrow gap between two working surfaces, which for this purpose are generally provided with working means in the form of ridges and intermediate grooves. Due to the rotation of one and, at times, both of said surfaces, the material is worked in the desired manner and transported out of the refiner by the forces of rotation. These refiners are generally of the disc-refiner type, but other types of refiners, such as the so-called conic refiners can also be used.
The intensity and kind of the treatment of the fibrous material is generally determined by the appearance or nature of the ridges and grooves on the working surfaces, and by the size of the gap therebetween. A certain gradual wear of the refining surfaces is unavoidable. For practical reasons, therefore, a disc refiner must be equipped with exchangeable refining elements, which are exchanged after a certain time when that wear has proceeded to a degree wherein it gives rise to process disturbances, or when the refining results are no longer satisfactory. The refining elements are also manufactured with a pattern and section in accordance with the kind of work to be carried out in the refiner. The energy transfer from these refining elements to the fibrous material is partly effected via the edges of the ridges and partly via the upper surfaces of the ridges.
The ridges, thus result in the fibrous material being subjected to compression and shearing forces. The highest compression forces are thus brought about when the material is clamped between the edges of the ridges, while the shearing forces are high when the material is rubbed between the surfaces of the ridges.
When these materials have disintegrated to fibrous level, i.e. when the fibers are exposed, the compression forces are most active. In order to develop the paperforming properties of the exposed fibers, a further treatment of the fibers is required. The object of this treatment is to split up or delaminate the exposed fibers. This treatment is best effected by shearing forces, because too high compression forces at this stage easily cause the fibers to break off, whereby the paper-forming properties of the refined material substantially deteriorate.
In the past, various refining element surfaces have been disclosed. Thus, U.S. Pat. No. 2,156,321 to Sutherland, Jr. discloses a fiber pulp refiner which principally employs a yielding or elastic surface, such as rubber. In addition, however, irrespective of the composition of these refining elements, the patentee discloses the use of distinct annular zones, such as those shown in FIGS. XI and XII. In particular, the patentee employs grooves which diminish in depth on the sharper outward paper then the portions of the grooves inside such zones. This may be seen in FIG. IX, for example.
Furthermore, U.S. Pat. No. 3,240,437 to Horstman discloses another refiner plate which, as shown in FIGS. 2 and 3, also employs convergent closely spaced bars 38. These bars increase in depth as one approaches the extremity of the plate. This is intended to relieve the compressive forces as they flow outwardly from the plates 25.
Finally, U.S. Pat. No. 2,035,994 to Sutherland, Jr. discloses yet another fiber refiner, and with particular reference to FIG. X thereof, again shows various annular sections on each refining element thereof. This patentee again discloses the diminution of the effective area and width of passage for the stock and its fibrous particals as they pass outwardly through these sections, such as by reducing the cross section and depth of the flow grooves thereof.
Each of these known refining surfaces have therefore proved deficient with respect to the above-noted objectives. It is therefore the object of the present invention to overcome these and other difficulties in prior art refining elements.
In particular, the present invention therefore teaches a refining element which in refining fibrous material yields a distribution of compression and shearing forces which is highly favorable for the paper-forming properties thereof.
According to the invention, it is therefore possible in a one-step refining process to achieve a pulp which is ready for paper-making.
The characterizing features of this invention will become apparent from the appended claims.